The segregation of a powder in a mixture of powders, whether of the same or different composition, has been a problem since earliest times. More recently, numerous mixtures of powders are shipped in bulk, particularly mixtures of pigments and certain finely divided filler materials. It has been found that where such finely divided powder mixtures are stored for a relatively long period of time, or where they are subjected to the expected vibrations of a journey from the point of shipment to the point of use, a pronounced segregation effect is noted. The manner in which a component of a mixture of powders may segregate is generally unpredictable, and the extent to which such segregation may occur is therefore difficult to estimate. Where segregation does occur, it is recognized that mixtures of finely divided powders often require remixing at the point of arrival prior to being used. Remixing is an economic handicap, and this unit operation is generally a source of irritation and danger due to the generally high proclivity of powder mixtures to dust. Our parent application taught a process for rendering normally high dusting powders dustless, and a process to prevent segregation of particles due to size or density differences. This process for prevention of segregation of solid particles was deemed particularly important when different powdered materials are to be homogeneously blended and thereafter maintained in a well-blended condition. The instant patent application more fully teaches a method of maintaining a mixture of finely divided particles having size and/or density differences in a homogeneous segregation-resistant state. By finely divided particles, we refer to particles generally smaller than about 100 mesh (U.S. Standard) though larger particles may also be prevented from segregation. The upper limit of the particle size affected by the process of this invention will depend on the particular physical properties of the particles.
A large number of prior art processes are directed to the prevention of segregation of finely divided materials. However, we are unaware of any reference teaching a process for prevention segregation of a powder mixture, whether dusty or not, without changing the essential physical characteristics of the mixture. The instant invention is directed to the prevention of segregation in a bulk, finely divided and superfinely divided mixture of particulate powders which, having been treated in accordance with the instant process, may be subjected to the vibrations inherent to the mode of transportation of the mixture, or stored practically indefinitely without segregating and without exhibiting dusting when used.
The process of this invention prevents segregation of finely divided materials whether they are dusty or not. It will be apparent that, though dustiness of a powder is not a prerequisite, the process will generally be of most benefit where the finely divided materials are normally high-dusting.
Materials which exhibit high dusting are exemplified by finely ground silica of less than 325 U.S. Standard mesh, finely ground pigments, flour, and various other inorganic and organic materials which have a sufficiently high content of fine and superfine particles to generate an undesirable dusting problem. A high tendency to dust is most evident in powder having particles in the micron and submicron range, as for example, in talc, clay, particulate electrodeposited metals and metal oxides formed in certain processes, and especially in carbon black and various forms of activated carbon.
Finely divided materials which are not normally high dusting include silicon carbide, coke, sand, heavy metal particles, and the like, which have a high tendency to segregate whether mixed with non-dusty powders or dusty powders.